A CMOS image sensor is an imaging device built with CMOS technology for capturing and processing light signals. Results produced by the CMOS image sensor can be displayed. A type of CMOS image sensors, called a CMOS Active Pixel Sensors (APS), has been shown to be particularly suited for handheld imaging applications.
The CMOS APS comprises an array of pixel processing elements, each of which processes a corresponding pixel of a received image. Each of the pixel processing elements includes a photo-detector element (e.g., a photodiode or a photogate) for detecting brightness information in the received image, and active transistors (e.g., an amplifier) for reading out and amplifying the light signals in the received image. The amplification of the light signals allows circuitry in the CMOS APS to function correctly with even a small amount of the received light signals.
The CMOS APS also has color processing capabilities. The array of pixel processing elements employs a color filter array (CFA) to separate red, green, and blue information from a received color image. Specifically, each of the pixel processing elements is covered with a red, a green, or a blue filter, according to a specific pattern, e.g., the “Bayer” CFA pattern. As a result of the filtering, each pixel of the color image captured by a CMOS APS with CFA only contains one of the three colors.
For example, while a given pixel may have data on how much red was received by that pixel, it does not have any data as to how much blue or green was received by that pixel. The “missing” values are recovered by a technique called interpolation whereby the values of each color for the surrounding pixels are averaged in order to estimate how much of that color was received by the given pixel.
While CMOS APSs have been well-received by industry and consumers alike, there are still some shortcomings. For example, as described above, each pixel contains a number of different parts required for capturing the image. The different parts are not ideal, of course, and can produce sensitivity variations over the array. With reference to FIG. 1, a CMOS APS contains a pixel array implemented in Si 100. The CMOS APS also contains a layer of protective Si oxide 105 which may also serve as a support for metal interconnects. The CMOS APS array further includes a color filter array 110 (e.g., a Bayer CFA) to allow only light of a specific wavelength to pass to each pixel within the active pixel area 100. The FIG. 1 CMOS APS also contains a layer of microlenses 115 that concentrates the incident light in the sensitive area of the underlying pixel and a main lens 120 that focuses the light rays 125 from the object onto the microlenses 115.
Most of the components described above, due to imperfections or practical limitations, may contribute to spatial signal attenuation, which in turn results in a sensitivity variation over the array. Further, it is known that for a given lens, the pixels of the APS have varying degrees of sensitivity depending upon their geographic location on the array. The rule of thumb is that the further away from the center of the APS the more correction the pixel requires. This phenomenon can adversely effect the images produced by the APS.
Often these variations can be measured and corrected as they mostly depend on the lens design used and generally do not vary from part to part. Such correction can be done in post-processing of already-acquired image data or during image acquisition (i.e., as the image is read out from the APS).
Since pixel sensitivity depends in part on the geometric location of a given pixel, generally speaking, one “global” correction function is not satisfactory. Prior knowledge of the non-uniform sensitivity of the pixels, when used with a particular type of lens, is used to generate a plurality of correction functions that are applied to (e.g., multiplied by) the pixel values as they are read out. In order to increase the special precision of the correction functions, the array is divided into a number of “zones,” where each zone includes a predetermined number of pixels and where the pixels of each zone are multiplied by a correction factor depending upon the zone and the pixel location relative to the APS center.
For example, a 640×640 pixel array may include 4 zones in the x-direction and 4 zones in the y-direction where each zone contains 128 rows or columns of pixels. Another example is to divide the APS array into a number of zones where the zones are configured to optimize a particular lens that is used. The boundaries of the zones, however, cannot be modified to accommodate any other lenses that may be used.
One disadvantage of the prior art is that the zones of known correction algorithms are fixed by design. That is, while a given non-uniform sensitivity correction algorithm may work well for a given type of lens, the algorithm does not work as well with another type of lens. Another disadvantage associated with the prior art is that when the center of the lens is not perfectly aligned with the center of the APS array, as is often the case, there is currently no method to take that offset into account and to adjust the zone boundaries for it.